Long-Term Application of Chemical and Organic Fertilizers over 35 Years Differentially Affects Interannual Variation in Soil Inorganic Phosphorus Fractions in Acidic Paddy Soil

Phosphorus plays an important role in double rice cropping systems grown on acidic paddy soil. To improve our knowledge and increase crop utilization of applied phosphorus fertilizers in paddy soils, we investigated changes in the content of soil inorganic phosphorus fractions in response to long-term application of chemical and organic phosphorus fertilizers. In this 35-year-long experiment, paddy soil in a double rice cropping system received one of three different fertilizer treatments: (1) cattle manure, M; (2) a chemical fertilizer of nitrogen (N), phosphorus (P), and potassium (K), NPK; and (3) a combination of NPK and M (NPKM) twice per year. Results showed that the maximum contents of Olsen-P in M, NPK and NPKM stabilized at 12.9, 31.7 and 52.7 mg/kg, respectively over 35 years. In contrast, the proportions of soil inorganic phosphorus content in total phosphorus of M, NPK and NPKM changed from 62.2% at the beginning of the experiment to 53.3, 61.9 and 66.2%, respectively. At equal amounts of accumulated phosphorus surplus, the increasing rates of total inorganic phosphorus content and inorganic phosphorus fractions in NPK were much higher than those in M and NPKM. At an average amount of accumulated phosphorus surplus of 100 kg/ha, the total inorganic phosphorus content of the NPK and NPKM treatments increased by 39.6 and 21.6 mg/kg, respectively. Fertilization mainly decreased the ratio of organic phosphorus to inorganic phosphorus and increased the ratio of aluminum-bound-phosphorus (Al-P) to inorganic phosphorus, especially under the NPK and NPKM treatments. Redundancy analysis showed that total phosphorus and Olsen-P were more closely correlated to iron-bound-phosphorus (Fe-P), calcium-bound-phosphorus (Ca-P) and Al-P fractions. This study suggests that the NPK and NPKM treatments increased phosphorus supply and inorganic phosphorus fractions compared to the manure application. Therefore, to avoid accumulation of a surplus of unabsorbed phosphorus and minimize phosphorus-leaching risk from acidic paddy soil, rates of inorganic phosphorus application, such as the combined application of manure and inorganic phosphorus fertilizer, should be reduced.